Lithography manufacturing has been the choice of volume production method for making electronic devices, such as semiconductor integrated circuit (IC), printed circuit board (PCB), light emitting diode (LED), laser diode (LD), flat panel display (FPD), quartz oscillators (QO), microelectromechanical (MEMS) parts and other electronic applications such as bump bonding, thin film heads (TFH) and multichip module manufacturing, 3D packaging using through silicon vias (TSV), and the related modem electronics and/or optical devices.
One of the lithography process challenges for volume manufacturing is to ensure a stable and consistent patterning performance within specifications from substrate to substrate, lot to lot, and day to day. The goal is to preserve a well-established process window to ensure profitable manufacturing yield while minimizing unwanted equipment downtime for the lithography tools used in manufacturing.
Conventionally, lithography technologies for manufacturing of modem electronic and optical devices are derived from lithography process technologies for making semiconductor ICs. Majority of lithography exposure tools used for patterning various types of substrates are contact or proximity aligners, projection stepper or scanner systems. Regardless of the tool types, depending on the process window control specifications, as general rule of thumb, it is desirable to keep the illumination intensity uniformity at about 1˜2% or less within the exposure field.
The reason for such a stringent illumination control requirement is mainly due to the image formation nature of photo resist that is highly dependent on the overall exposure energy that was received. For an actinic exposure wavelength, the exposure energy (mili-Joules or mJs) is the product of optical intensity (mili-watts/sq-cm) and time (seconds). To control the uniformity of the line width for the printed feature to be within specifications, the first consideration is to ensure illumination intensity to be consistent and stable. With a higher intensity level, for nominal exposure energy, the exposure time can be shorter, and the exposure throughput can be better. Should the illumination intensity become lower, to get the same nominal exposure energy, it can be compensated by increasing the exposure time. Hence for a typical lithography manufacturing with nominal exposure energy, it is desirable to monitor the optical intensity level.
Traditionally lithography exposure tool has been designed with a single illumination source, such as using a mercury short-arc lamp or an excimer laser. There is a limited life time for mercury arc lamp to be several hundred hours. For excimer laser, the gas used for laser emission must be re-filled in a year or less of operation. Since the life time and failure mode of both source types have been well-characterized, it has been relatively straightforward to monitor optical intensity with a single illumination source.
Without a viable alternative in the past, the industry has accustomed to limited life time and excessive electrical energy used in mercury arc lamp. In order to improve the exposure throughput, more optical intensity must be generated. Typically for mercury arc lamp specified with 1K Watts input electrical power, to generate the desired actinic exposure wavelength, say, 365 nm, by passing through an in-line optical filter. Depending on the optical system used, it can typically deliver 100˜200 m Watts or thereabout optical intensity at the photo resist surface. For 100 mJs nominal exposure energy, it may use 0.5 to 1 seconds of exposure time.
As the lamp becomes aged, the actinic optical power is decreased, more exposure time is used. Eventually for several hundreds of hours in operation, the lamp output becomes too low or simply failed for no output. Such a lithography manufacturing using mercury arc lamp, the typical practice of performing illumination monitoring process is to check optical intensity level daily. For each process lot, a test exposure is performed. After resist develop, the feature line width is measured against a specification range. Then it is either to tune the exposure time or change to the exposure energy setting to print the target line width.
The replacement of high pressured mercury arc lamp source is not just a quick lamp change process. The lamp must be tuned off and let it cool down for handling. After a new lamp is installed, the arc source must be focused and adjusted to optimize for the best intensity and uniformity. The task normally takes hours to accomplish before allowing the tool to resume manufacturing. Every two to four weeks in operation, the same task must be repeated for lamp replacement. Unlike mercury arc lamp where the main optical output bands are from near UV to visible, for excimer laser, however, the optical output wavelength is in deep UV regions such as either 248 nm or 193 nm. The excimer illumination system cost is in the million US dollar range. The illumination maintenance is much more elaborated and could take days. Typically it is required to perform excimer gas re-filling and tuning either semiannually or annually.
Therefore, there is a need to address the above issues of the conventional illumination systems.